Bash loop until a certain command stops failing - bash

Question

I would like to write a loop in bash which executes until a certain command stops failing (returning non-zero exit code), like so:
while ! my_command; do
# do something
done
But inside this loop I need to check which exit code my_command returned, so I tried this:
while ! my_command; do
if [ $? -eq 5 ]; then
echo "Error was 5"
else
echo "Error was not 5"
fi
# potentially, other code follows...
done
But then the special variable ? becomes 0 inside the loop body.
The obvious solution is:
while true; do
my_command
EC=$?
if [ $EC -eq 0 ]; then
break
fi
some_code_dependent_on_exit_code $EC
done
How can I check the exit code of my_command (called in loop header) inside loop body without rewriting this example using a while true loop with a break condition as shown above?

Answer 1

In addition to the well-known while loop, POSIX provides an until loop that eliminates the need to negate the exit status of my_command.
# To demonstrate
my_command () { read number; return $number; }
until my_command; do
if [ $? -eq 5 ]; then
echo "Error was 5"
else
echo "Error was not 5"
fi
# potentially, other code follows...
done

Answer 2

If true command hurt your sensibility, you could write:
while my_command ; ret=$? ; [ $ret -ne 0 ];do
echo do something with $ret
done
This could be simplified:
while my_command ; ((ret=$?)) ;do
echo do something with $ret
done
But if you don't need ResultCode, you could simply:
while my_command ; [ $? -ne 0 ];do
echo Loop on my_command
done
or
while my_command ; (($?)) ;do
echo Loop on my_command
done
And maybe, why not?
while ! my_command ;do
echo Loop on my_command
done
But from there you could better use until as chepner suggest

Answer 3

You can get the status of a negated command from the PIPESTATUS built-in variable:
while ! my_command ; do
some_code_dependent_on_exit_code "${PIPESTATUS[0]}"
done
chepner's solution is better in this case, but PIPESTATUS is sometimes useful for similar problems.

Answer 4

So in my case I also need to ignore some exit codes and want to provide some useful output to the user so I wrote this up:
retrycmd(){
MSG=$1
IGNORE=$2
shift 2
local SLEEP_T=5
local L_CNT=5
local C_CNT=0
while ((C_CNT++ < ${L_CNT})) && ! $#;do
RET=${PIPESTATUS[0]}
#echo "RET: ${RET}"
for I in ${IGNORE//,/ };do # bashism: replace(/) all(/) match(,) with(/) value(<space>)
if ((${RET} == ${I}));then
#echo "${RET} = ${I}"
break 2
fi
done
echo "${MSG} failure ${C_CNT}"
sleep ${SLEEP_T}
done
if ((${C_CNT} > ${L_CNT}));then
echo "${MSG} failed"
poweroff
fi
}
#retrycmd "Doing task" "IGNORE,CSV" <CMD>
retrycmd "Ping google" "0" ping www.google.com